Welding applications, such as pipe welding, often require high currents and use several arcs created by tandem electrodes. Such welding systems are quite prone to certain inconsistencies caused by arc disturbances due to magnetic interaction between two adjacent tandem electrodes. A system for correcting the disadvantages caused by adjacent AC driven tandem electrodes is disclosed in Stava U.S. Pat. No. 6,207,929. In that prior patent, each of the AC driven electrodes has its own inverter based power supply. The output frequency of each power supply is varied so as to prevent interference between adjacent electrodes. This system requires a separate power supply for each electrode. As the current demand for a given electrode exceeds the current rating of the inverter based power supply, a new power supply must be designed, engineered and manufactured. Thus, such system for operating tandem welding electrodes require high capacity or high rated power supplies to obtain high current as required for pipe welding. To decrease the need for special high current rated power supplies for tandem operated electrodes, assignee developed the system disclosed in Stava U.S. Pat. No. 6,291,798 wherein each AC electrode is driven by two or more inverter power supplies connected in parallel. These parallel power supplies have their output current combined at the input side of a polarity switching network. Thus, as higher currents are required for a given electrode, two or more parallel power supplies are used. In this system, each of the power supplies are operated in unison and share equally the output current. Thus, the current required by changes in the welding conditions can be provided only by the over current rating of a single unit. A current balanced system did allow for the combination of several smaller power supplies; however, the power supplies had to be connected in parallel on the input side of the polarity reversing switching network. As such, large switches were required for each electrode. Consequently, such system overcame the disadvantage of requiring special power supplies for each electrode in a tandem welding operation of the type used in pipe welding; but, there is still the disadvantage that the switches must be quite large and the input, paralleled power supplies must be accurately matched by being driven from a single current command signal. Stava U.S. Pat. No. 6,291,798 does utilize the concept of a synchronizing signal for each welding cell directing current to each tandem electrode. However, the system still required large switches. This type of system was available for operation in an ethernet network interconnecting the welding cells. In ethernet interconnections, the timing cannot be accurately controlled. In the system described, the switch timing for a given electrode need only be shifted on a time basis, but need not be accurately identified for a specific time. Thus, the described system requiring balancing the current and a single switch network has been the manner of obtaining high capacity current for use in tandem arc welding operations when using an ethernet network or an internet and ethernet control system. There is a desire to control welders by an ethernet network, with or without an internet link. Due to timing limitation, these networks dictated use of tandem electrode systems of the type using only general synchronizing techniques.
Such systems could be controlled by a network; however, the parameter to each paralleled power supply could not be varied. Each of the cells could only be offset from each other by a synchronizing signal. Such systems were not suitable for central control by the internet and/or local area network control because an elaborate network to merely provide offset between cells was not advantageous. Houston U.S. Pat. No. 6,472,634 discloses the concept of a single AC arc welding cell for each electrode wherein the cell itself includes one or more paralleled power supplies each of which has its own switching network. The output of the switching network is then combined to drive the electrode. This allows the use of relatively small switches for polarity reversing of the individual power supplies paralleled in the system. In addition, relatively small power supplies can be paralleled to build a high current input to each of several electrodes used in a tandem welding operation. The use of several independently controlled power supplies paralleled after the polarity switch network for driving a single electrode allows advantageous use of a network, such as the internet or ethernet.
In Houston U.S. Pat. No. 6,472,634, smaller power supplies in each system are connected in parallel to power a single electrode. By coordinating switching points of each paralleled power supply with a high accuracy interface, the AC output current is the sum of currents from the paralleled power supplies without combination before the polarity switches. By using this concept, the ethernet network, with or without an internet link, can control the weld parameters of each paralleled power supply of the welding system. The timing of the switch points is accurately controlled by the novel interface, whereas the weld parameters directed to the controller for each power supply can be provided by an ethernet network which has no accurate time basis. Thus, an internet link can be used to direct parameters to the individual power supply controllers of the welding system for driving a single electrode. There is no need for a time based accuracy of these weld parameters coded for each power supply. In the preferred implementation, the switch point is a “kill” command awaiting detection of a current drop below a minimum threshold, such as 100 amperes. When each power supply has a switch command, then they switch. The switch points between parallel power supplies, whether instantaneous or a sequence involving a “kill” command with a wait delay, are coordinated accurately by an interface card having an accuracy of less than 10 μs and preferably in the range of 1–5 μs. This timing accuracy coordinates and matches the switching operation in the paralleled power supplies to coordinate the AC output current.
By using the internet or ethernet local area network, the set of weld parameters for each power supply is available on a less accurate information network, to which the controllers for the paralleled power supplies are interconnected with a high accuracy digital interface card. Thus, the switching of the individual, paralleled power supplies of the system is coordinated. This is an advantage allowing use of the internet and local area network control of a welding system. The information network includes synchronizing signals for initiating several arc welding systems connected to several electrodes in a tandem welding operation in a selected phase relationship. Each of the welding systems of an electrode has individual switch points accurately controlled while the systems are shifted or delayed to prevent magnetic interference between different electrodes. This allows driving of several AC electrodes using a common information network. The Houston U.S. Pat. No. 6,472,634 system is especially useful for paralleled power supplies to power a given electrode with AC current. The switch points are coordinated by an accurate interface and the weld parameter for each paralleled power supply is provided by the general information network. This background is technology developed and patented by assignee and does not necessarily constitute prior art just because it is herein used as “background.”
As a feature of the system in Stava U.S. Pat. No. 6,207,929, two or more power supplies can drive a single electrode. Thus, the system comprises a first controller for a first power supply to cause the first power supply to create an AC current between the electrode and workpiece by generating a switch signal with polarity reversing switching points in general timed relationship with respect to a given system synchronizing signal received by the first controller. This first controller is operated at first welding parameters in response to a set of first power supply specific parameter signals directed to the first controller. There is provided at least one slave controller for operating the slave power supply to create an AC current between the same electrode and workpiece by reversing polarity of the AC current at switching points. The slave controller operates at second weld parameters in response to the second set of power supply specific parameter signals to the slave controller. An information network connected to the first controller and the second or slave controller contains digital first and second power supply specific parameter signals for the two controllers and the system specific synchronizing signal. Thus, the controllers receive the parameter signals and the synchronizing signal from the information network, which may be an ethernet network with or without an internet link, or merely a local area network. The invention involves a digital interface connecting the first controller and the slave controller to control the switching points of the second or slave power supply by the switch signal from the first or master controller. In practice, the first controller starts a current reversal at a switch point. This event is transmitted at high accuracy to the slave controller to start its current reversal process. When each controller senses an arc current less than a given number, a “ready signal” is created. After a “ready” signal from all paralleled power supplies, all power supplies reverse polarity. This occurs upon receipt of a strobe or look command each 25 μs. Thus, the switching is in unison and has a delay of less than 25 μs. Consequently, both of the controllers have interconnected data controlling the switching points of the AC current to the single electrode. The same controllers receive parameter information and a synchronizing signal from an information network which in practice comprises a combination of internet and ethernet or a local area ethernet network. The timing accuracy of the digital interface is less than about 10 μs and, preferably, in the general range of 1–5 μs. Thus, the switching points for the two controllers driving a single electrode are commanded within less than 5 μs. Then, switching actually occurs within 25 μs. At the same time, relatively less time sensitive information is received from the information network also connected to the two controllers driving the AC current to a single electrode in a tandem welding operation. The 25 μs maximum delay can be changed, but is less than the switch command accuracy.
The unique control system disclosed in Houston U.S. Pat. No. 6,472,634 is used to control the power supply for tandem electrodes used primarily in pipe seam welding and disclosed in Stava U.S. Pat. No. 6,291,798. This Stava patent relates to a series of tandem electrodes movable along a welding path to lay successive welding beads in the space between the edges of a rolled pipe or the ends of two adjacent pipe sections. The individual AC waveforms used in this unique technology are created by a number of current pulses occurring at a frequency of at least 18 kHz with a magnitude of each current pulse controlled by a wave shaper. This technology dates back to Blankenship U.S. Pat. No. 5,278,390. Shaping of the waveforms in the AC currents of two adjacent tandem electrodes is known and is shown in not only the patents mentioned above, but in Stava U.S. Pat. No. 6,207,929. In this latter Stava patent, the frequency of the AC current at adjacent tandem electrodes is adjusted to prevent magnetic interference. All of these patented technologies by The Lincoln Electric Company of Cleveland, Ohio have been advances in the operation of tandem electrodes each of which is operated by a separate AC waveform created by the waveform technology set forth in these patents. These patents are incorporated by reference herein. However, these patents do not disclose the present invention which is directed to the use of a unique implementation of waveform technology to create a specific waveform for use in welding using an AC current and a cored electrode.
When using the waveform technology as so far described for off-shore welding or welding on pipelines, the welding process generally used solid welding wires with a shielding gas. In this type of process, DC welding as described, together with STT welding has been the normal practice. When cored electrodes are used, the core can be formed of alloy material to make the weld metal. Such processes generally required DC welding using cored electrodes. Consequently, in the past cored or solid wire using a DC process with external shielding gas has been the normal practice, especially for off-shore welding and pipeline welding. The DC welding presented little problems of uneven burn back of the sheath and core. The electrodes were cored for alloying. The need for controlled strength and hardness combined with low diffusible hydrogen limits made it difficult to use AC welding. These DC welding processes have been used in the field and are the background to which the present invention is directed. There has been no use of cored electrodes and AC welding because the AC waveforms were not tailored to any particular cored electrode. The burn rate for the sheath and core could not be controlled.